1 =================
2 Linux I2C and DMA
3 =================
4
5 Given that I2C is a low-speed bus, over which
6 transferred are small, it is not considered a
7 time of writing, only 10% of I2C bus master dr
8 implemented. And the vast majority of transact
9 DMA for it will likely add more overhead than
10
11 Therefore, it is *not* mandatory that the buff
12 It does not seem reasonable to apply additiona
13 rarely used. However, it is recommended to use
14 message size is likely applicable for DMA. Mos
15 around 8 bytes (as of today, this is mostly an
16 any message of 16 byte or larger, it is probab
17 note that other subsystems you use might add r
18 I2C bus master driver is using USB as a bridge
19 safe buffers always, because USB requires it.
20
21 Clients
22 -------
23
24 For clients, if you use a DMA safe buffer in i
25 flag with it. Then, the I2C core and drivers k
26 on it. Note that using this flag is optional.
27 updated to use this flag will work like before
28 using an unsafe DMA buffer. To improve this si
29 more and more clients and host drivers is the
30 that setting this flag makes only sense in ker
31 copied into kernel space anyhow. The I2C core
32 buffers in kernel space are always DMA capable
33 SMBus transactions via I2C, the buffers for bl
34 of i2c_master_send() and i2c_master_recv() fun
35 variants (i2c_master_send_dmasafe() and i2c_ma
36 know their buffers are DMA safe. Users of i2c_
37 I2C_M_DMA_SAFE flag manually.
38
39 Masters
40 -------
41
42 Bus master drivers wishing to implement safe D
43 the I2C core. One gives you a DMA-safe buffer
44 certain threshold is met::
45
46 dma_buf = i2c_get_dma_safe_msg_buf(msg
47
48 If a buffer is returned, it is either msg->buf
49 bounce buffer. But you don't need to care abou
50 returned buffer. If NULL is returned, the thre
51 buffer could not be allocated. Fall back to PI
52
53 In any case, a buffer obtained from above need
54 function ensures a potentially used bounce buf
55
56 i2c_put_dma_safe_msg_buf(dma_buf, msg,
57
58 The last argument 'xferred' controls if the bu
59 message or not. No syncing is needed in cases
60 there was no data transferred.
61
62 The bounce buffer handling from the core is ge
63 allocate a new bounce buffer. If you want a mo
64 reusing pre-allocated buffers), you are free t
65
66 Please also check the in-kernel documentation
67 driver can be used as a reference example how
68
69 Final note: If you plan to use DMA with I2C (o
70 make sure you have CONFIG_DMA_API_DEBUG enable
71 you find various issues which can be complex t
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.